1. Field of the Invention
The present invention relates to a structure of a metal mask for depositing a deposit with the metal mask intimately held on a substrate and to a method for manufacturing thereof.
2. Description of the Related Art
In an organic electroluminescent device (an organic EL device) used for such as a display device or a flat panel display, electrons which are injected from a cathode and holes which are injected from an anode are recombined within an organic fluorescent dye interposed between both electrodes, in order to excite the dye for obtaining the luminescence. Therefore, this device attracts attention because this device has some excellent characteristics as compared with a liquid crystal display (LCD), that is, for example, an angle of visibility is large, a high contrast can be easily realized at the high brightness, miniaturization of the device can be achieved since a back light is not required due to the employment of the spontaneous emission of the dye and an extremely thin panel having a thickness below two millimeters can be realized, and further, this device is suitable for an application such as dynamic images reproduction because its response time is much shorter than that of the LCD.
Colorization of such an organic EL device is also under scrutiny, for example, a parallel type independent system in which three colors of RGB pixels are formed of different luminescent layers containing different dyes respectively, a color conversion system in which the light generated from one kind of blue luminescent layer is converted to three colors of RGB after passing through a fluorescent color conversion film, and a color filter system in which the light from a white luminescent layer is passed through a color filter for obtaining the three colors of RGB are known.
In the color conversion system or in the color filter system, only one luminescent layer is needed, so that the patterning of the luminescent layer is not required. Also, a color conversion film or a color filter which requires to be patterned can be achieved by a conventional lithographic method, but there is a problem that a luminous efficiency decreases through the color conversion film or the color filter.
On the other hand, the parallel type independent system is advantageous as compared with other types, because this system has a characteristic that the luminous efficiency is excellent due to the unnecessity of the color conversion film or the color filter. However, it is necessary to coat a fine luminescent layer for every each color and a high performance material is required to form each of the three colors of luminescent layers. In particular, an organic dye used for the luminescent layer is poor in resistance to moisture or organic solvents, so that patterning through a wetting process which is a representative of the photolithographic method is difficult to be performed. In addition, as for the electrodes which are formed on the organic luminescent layer, the organic dye is adversely affected by performing the pattern processing through the wetting process, so that in both cases, the dry process such as deposition is used and the patterning has been achieved by using a mask.
However, when a fine pattern is formed, a mask should be made thinner; otherwise the film thickness of a deposit around an opening becomes thinner, so that a deposit having an uniform thickness can not be obtained. Further, the deposition has to be performed with the mask intimately contacted to a substrate in order to perform the deposition with high precision. However, as the mask becomes thinner, the mask tends to be bent, so that a gap is formed between the mask and the substrate. Therefore, especially at a portion which is closer to the central portion, the gap becomes larger and the deposition pattern tends to be blurred. In addition, an increase in temperature causes expansion of the mask during the deposition and the thinner layer results in lack of mechanical rigidity, so that a slight vibration and stress result in to displacement of a line position of the mask, and the alignment with the metal mask becomes difficult to be performed in particular when multi-color luminescent layers are deposited. As described above, there has been a problem of how to ensure its precision.
In actual manufacturing, a plurality of EL device patterns are deposited at a time on one substrate, then the patterns are divided to an individual EL device. That is, the plurality of EL device patterns are deposited at a time on a large-sized glass substrate, so that the mask also becomes larger, and the mask tends to be further deflected. Therefore, the frame has conventionally been laid like a beam between EL device patterns and the metal mask has been welded and secured to the frame in order to resolve the deflection of the mask. In addition, it is an effective method as disclosed in a Japanese Patent Application Laid-Open No. 10-41069 that the mask is secured to the frame with tension applied thereto in order to allow the flatness of the mask to be in its ideal state.
Although various methods such as laser welding and spot welding are taken for welding the mask to the frame, a welding flash having a height of several μms to several hundreds of μms is generated on a mask surface which is contacted to the substrate during welding. As shown in FIG. 4(a), a typical deposition method is performed by torating the substrate with an evaporation source 48 displaced from a center position of the substrate 50. However, when the deposition is performed with this welding flash remained as it is, the welding flash 43 protrudes at a welding portion between the frame 41 and the mask 42 as shown in FIG. 4(b), so that a gap 44 is created between the substrate 45 and the mask 42. For example, when a luminescent layer 47 is deposited from one evaporation source 48 as shown in the same Figure, a deposition incidence angle at the farthest end of the deposition area from the evaporation source 48 and a deposition incidence angle at the nearest end of the deposition area from the evaporation source 48 become different. The deposition incidence angle at the farthest end of the deposition area is minimized, and at the farthest side from the center of the substrate, the deposition is performed in a larger area than a predetermined area (the same width as that of ITO). Further, it is necessary to allow a uniform part of the luminescent layer to correspond to the ITO electrode, but the uniform part may not correspond to the ITO electrode because a center of the uniform part of the luminescent layer which is formed at the nearest position from the evaporation source (deposition incidence angle is maximum (52)) and a center of the ITO electrode are displaced from each other due to the fact that the position which is closer to the center of the substrate is shaded with a slit. Further, when the luminescent layer is required to be coated for every each color in order to achieve full-colorization, the layer is formed with the same mask displaced by a pitch of ITO. However, at the end of the deposition area, the layer is deposited onto an adjacent pixel portion (an adjacent ITO electrode 46) at the farthest position from the center of the substrate. In particular, this tendency becomes significant as the pitch and space become narrower.
In case of adopting a method in which the welding flash is removed by such as grinding, welding peeling occurs by rubbing motion during the grinding and the mask will be deflected by radiation heat during the deposition, so that it may be impossible to form the fine patterns. In addition, when the mask is secured to the frame with the tension applied thereto, the pattern per se might often be deformed by the welding peeling. Therefore, it has been difficult to solve the above described problems since the deposition has usually been performed with the welding flash remained as it is.